Sodium chloride tends to form large, agglomerated masses upon exposure to moisture, particularly during long periods of storage. These hardened masses are generally referred to as cakes. A non-caking agent is often added to the salt to prevent the formation of cakes. Sodium or potassium ferrocyanide is often used as a non-caking additive. However, a major disadvantage of these compounds is that they contain nitrogen. The presence of nitrogen in salt compositions is highly undesired, because when the salt is used in electrolysis operations, explosive NCl3 will be obtained. Another disadvantage of commercially used ferrocyanide is the fact that the iron introduced by this agent is very difficult to remove from brine produced from salt containing said anti-caking agent. Especially if the brine is used in membrane electrolysis cells, the ferrocyanide that is introduced will disintegrate in the cell and the free iron will precipitate, typically in the form of the hydroxide, in and on the membrane. This will lead to less efficient membrane electrolysis operations. Further, there is an ongoing debate in respect of the desirability of sodium or potassium ferrocyanide in table salt.
In recent years much effort has been put into the development of improved non-caking salt agents which are inexpensive and environmentally safe, and which are effective in small amounts. WO 00/59828, for instance, describes the use of a metal complex of a hydroxypolycarboxylic compound as a non-caking agent in salt compositions. Complexes of iron with these hydroxypolycarboxylic acids were found to render salt non-caking at low concentrations. It is described that the use of tartaric acid has particular advantages. Further, it is described that meso-tartaric acid is the most preferred anti-caking agent. However, although pure meso-tartaric acid is commercially available, prices are much too high for application in a non-caking additive for sodium chloride on industrial scale.
WO 00/59828 discloses that besides an iron, titanium and/or chromium complex of meso-tartaric acid also a mixture of hydroxypolycarboxylic acids wherein at least 5% by weight of the hydroxypolycarboxylic acid is meso-tartaric acid can be used as non-caking additive for sodium chloride. In this respect, WO 00/59829 discloses a method for producing a mixture of tartaric acid which includes meso-tartaric acid. It mentions that it can be prepared by treating a natural or synthetic tartaric acid (CAS registry numbers 87-69-4 and 147-71-7, respectively) solution with concentrated NaOH at temperatures above 100° C. Part of the L-, D-, and/or DL-tartaric acid is then converted to the desired meso-tartaric acid (CAS registry number 147-73-9). It was found that by following this procedure, it is merely possible to prepare a mixture of tartaric acid with up to a maximum of 50% by weight being the meso isomer.
Electrolyser membranes and electrodes are extremely sensitive to impurities in the brine, particularly multivalent metal ions such as iron. The multivalent metals tend to precipitate inside the membrane, causing irreversible damage to the membrane. Not only inorganic contaminants form a problem. Also organic compounds present in the brine may cause problems. When brine solution containing organic contaminants is electrolyzed, the organic species may precipitate on the surface of and within an electrolysis cell membrane causing plugging. Some organic compounds, such as tartaric acid, do not give precipitate problems because they are broken down to harmless CO2. However, this CO2 ends up in certain downstream products. When a boundary level is exceeded, this CO2 gives rise to purity problems. Hence, to achieve maximum life-time of a separator in an electrolysis cell and to avoid purity problems, the concentration of organics and multivalent metal cations such as iron should be reduced to as low a level as possible in the feed-brine solution.
With compositions according to Example 1 of WO 00/59828 wherein meso-tartaric acid is used in combination with hydropolycarboxylic acids such as DL-tartaric acid, relatively high amounts of organics are introduced on the salt. On the other hand, using commercially available 100% meso-tartaric acid, even when used in admixture with racemic DL-tartaric acid or L-tartaric acid, is economically not feasible because of its high price.